As with any other vSphere setup, i am using VMware Workstation to run my entire VSAN Lab. I have installed VCSA 6.5 on my workstation as an appliance and have three ESXi 6.5 hosts installed as workstation VMs. I have connected two ESXi hosts directly using the LAN Segment to mimic the setup of 2 Node VSAN cluster. Also i i have added additional NICS as needed to enable vMotion and configuring redundancy for Management Network. Any configuration works as long as there is proper connectivity and VMkernel interfaces are enabled with VSAN Traffic.
As discussed earlier, the Witness Host cannot be part of same VSAN cluster and canno tbe part of another Stretched or 2 Node VSAN cluster. Special configurations are supported by VMware with RPQ and you may need to get in touch with support for further details.
Add the ESXi hosts to vCenter and configure networking as below and ensure there are VMkernel interfaces enabled with VSAN Traffic on each host. Since we are doing a 2 Node Direct Connect setup, configure the vmkernel interfaces with different subnet. In my case i am using 188.8.131.52 network.
Hope this was informative. Thanks!